Inhibiting polymerization during preparation of synthetic resin sheets



Aug. 19, 1952 R, s TAYLOR 2,607,081

INHJ-.BITING POLYMERIZATION DURING PREPARATION OF' SYNTHETIC RESINSHEETS Filed April 25, 1951 m fjmilzzfil N v 1N VEN TOR.'

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ATT RNEY.

optically perfect sheets.

Patented Aug. 19, 1952 INHIBITNG POLYMERIZATION DURING PREPARATION OFSYNTHETIC RESINl Y SHEETS;

Robert S. Taylor,lJest Chester, Pa., assigner to E. I. du Pont deNemours and Company, Wil-l minatori, Del., a corporation of Delawareelimination April 23, 1951, serial No. 222,416

3 Claims. 1

This invention relates to the preparation of synthetic resin sheets andmore particularly to a continuous process of casting methyl methacrylatesheets. v. f

The R. T.' Fields U. S. Patent 2,537,970, issued January 16, 1951,describes a process of polymerizing methyl methacrylate by distributingthe monomeric ester, usually containing some polymer, upon the surfaceof a body of an aqueous salt solution which is substantially saturatedat operating temperatures. The process is carried out by initiating thepolymerization of the monomeric ester with actinic energy althoughthermal energy may be used if temperatures are maintained below 80 C.Clearpolymeric sheets of polymethyl methacrylate, polystyrene, etc., areproduced when aqueous solutions of the preferred salts, such asmagnesium chloride and lithium chloride, are used. The Fields process isa long step in advance over previous techniques of producing polymericsheeting such as that described in the Rohm et al. U. S. Patent2,154,639, in which polymerization is carried out in molds or cellscomposed of two sheets of glass separated by a compressible gasket. Byfollowing the process of the Fields patent supra and the processdisclosed in the instant case, superior polymeric resin sheet of opticalquality may be obtained.

In carrying out the general Fields process of continuously preparingsheets of polymeric methyl methacrylate and other polymers, a number ofproblems have 'been encountered in producing imperfection-free sheetsand especially It was found that during polymerization of the organiccompound-contact of that compound with stationary solid surfaces couldnot be tolerated. Moreover, the upper exposed surface of the sheet ofpolymerizing organic compound must polymerize at substantially the samerate as the lower surface thereof and should not, it has been found, beinA contact with substances that inhibit polymerization. Furthermore,care must be exercised in order to prevent polymerization of smallamounts of liquid monomer which may have dissolved in the supportingliquid.

An object of the present invention is to p rovide a process of preparingsynthetic resin sheets of substantially 'optical clarity. Another objectis to provide a process of distributing a liquid polymerizable organiccompound upon the surface of a moving body of an aqueous salt solutionin a uniform and continuous manner. A further object is to provide amethod of eliminating, during the polymerization of vmethyl meth- Zacrylatesheeting, causes of optical distortion. Othery objects andadvantages of the invention will hereinafter appear.

The invention relates in general to thepolymerization` of a Amonomericorganic compound by distributing a compound as4 a liquid layer on thesurfaceof a body of an aqueous salt solution and thereafter polymerizingthe Vcompound at a temperature between 0 and 80C., the aqueous saltsolution being maintainedbetween C. and 80.C.,-being substantiallysaturated with a salt at said'temperature and'having a Yspecic gravityat least as great as the polymer being i formed. The polymerizationprocess is conduct- `the drawings which diagrammatically illustrate aform of apparatus that may be used and by reference to the examplesillustrating preferred embodiments of the invention.

Figure '1 is a diagrammatical plan view of an apparatus for carrying outthe invention in which retaining belts are shown for confining thedistrbution-of theA organic vcompound on the surface ofxthe saltsolution.Y f v Figure 2 isa section E-Ei of the apparatus shown inFigure l. i

The apparatus shown in Figures 1 and 2Comlprises in combination tray 'Avof'Monel-metal or other ,suitable corrosion-resisting metal `into Whichis introduced an aqueous Ysalt solution throughinlet l, the'solutionnowing continuously over vweir la'into tray Aand overflowingcontinuouslyl overi'the dam Ib being discharged through pipe 2.' A syrupof monomeric and polymeric -methyl'methacrylate is poured upon thesurface of the aqueous salt solution through inlet 'pipe 3."Syrup-retaining barriers, such as belts 4,

fabricated from polytetrafluoroethylene, or other suitable material,retain the syrup upon-the `surface of the aqueous salt solution, thesyrup moving concurrently with the barrier. and the aqueous vsaltsolution. A bank of fluorescent lights B, producing light of maximumintensity at approximately 3600 Angstroms wavelength, is suspended abovethe casting unit and serves as the source of energy for initiatingpolymerization. Upon substantial completion of polymerization, theiinished sheet C is continuously withdrawn from the unit. The entireunit is closed to the atmosphere by covering the top of the castingtraywith glass 5 or other suitable material, and the end of the unit, Wherethe sheet is Withdrawn, is sealed from the atmosphere by naps 6 ofrubber or other suitable flexible material Which-@actas the exit seal.Prior to operationof' the' unit; the air in space D is replaced withnitrogen, or other inert gas, and during polymerization the inert gas isintroduced into thesealed Vunit through header l, the gas iowingoverthe; substantially completely polymerized portion of the sheet toreduce the concentration cf'methyl-methacrylate vapor. The gas leavesthe unit throughtheexitr seal under iiaps 6.

The barrier used forrestricting the lateral flow of thepolymerizing'resinon the'concentrated salt solution maybe a verticallydisposed'fflexiblebelt, link belt or any other 1suitable"retainingmeans.

Vfis-shown in Figure *1, two endless -beltsare used, y the belts beingadvanced at the desired'rateby vdriven pulleys I0, operated 'bya powersource not shown. The vbelts are moved through a-path in `front of theMonel metal guides Ill--andlaround idling' pulleys l2. Themonomer-polymer lsyrup flows over the salt solution, forms'a layerconfined by thebelt 4, advances at substantially' the-same Afrate as thebelt, polymerizesas'it -adVa-ncesand asgthe polymerized sheet leaves thetray andthe jsuppori'fo the salt' solution held'A therein; it passes.

overfidler-*rolls I Sand i4 and-above therubber exitflexible walls` ll6.

The dotted line 8 inFigur@ -1 il-lustratesthe-approximate dividing linebetween substantially completelypolymerized material 'and thepolymerizingmass of material. It is important for optimum results,forreasons described `hereinafter, that the inert gas-be introducedVinto Vthe -unit at a point immediately rbeyond this dividing line ontheside ofthe polymerized-'portiom the ',-gas'ilowing inthe ldirectiondesignated by the arv:immediately .beyond the; division., "between: Vthe"polymerizing mass and-f the substantially -solid polymerized sheet-sothat theflow'ofgas is toward theexitzend ofthe unit. Ifthegas-Were'permitted toflow'over the'entire lengthpf the unit,;thatisi-over both the liquidfpolymeriz-ingmass and the substantially solidportion of the-.sheetthe :current .off gas would promote' evaporationof; liquid polymerizablematerial,fromrthe liquid ,polymerizingfmass.Moreover,-suchkevaporation'wouldzdisruptrthe homogeneity of thepolymerizableiliquid, lespleciallyiwhen a syrup of polymer Adissolved'inmonomer is used, :andlthis would resultk in the production ofsheetinghaving opticalV distortions.

Furthermore, if an inert gas were-permitted to flow 'over the entirelength ofthe unit, evaporationlofvpolymerizable liquid into the gas`would saturate the gas with vapor of polymerizable liquid, and thissaturated atmosphere would cause pitting of the polymerized sheet at theexit end of the unit. Hence, an inert substantially stagnant atmosphereis maintained above the upper surfaceof the polymerizing masspermittingthe inert gas to iiow only across the upper surface of the substantiallypolymerized sheet.

The following example illustrates embodiments ofthe invention in whichparts are by weight unless otherwise stated.

An aqueous solution of magnesium chloride was prepared, saturated atabout 10 C., and having-'aespecific gravity of about 1.3 at thattemperattire. :Sodiumnitrite was added to the solution, Vthe quantitybeing sucient to produce a concentration of about 0.001%, based upon thetotal Weight of the aqueous magnesium chloride solution; The resultingsolution was poured into a Monel metal tray 5 wide by 28 long by 6"deep, until the solution rose toa depth of` about 11/2.

A syrup of about methyl methacrylate `Irionomerandfabout 15% of itspolymer byweight, having a viscosity of about 15 poises andrcontainringabout 0.8% ofiwatenbased uponthe Weight of syrup, and containing 013 ofalphagalphaazodiisobutyronitrile, based upon the Weight of syrup,Waspoured Aon the salt solution through nozzle 3a. If desired, `a 'syrupcontaining greater or lesseramounts of the polymer-'dissolved in themonomeric ester may be usedv and with careful controlof polymerizationtemperature, ,pure monomer'can be used. A'pairof retaining belts; 0.05inch in thickness by zinchesin width, fabricated frompolytetrafluoroethylene, Were'arrangedgin a position `similar to thatshown` in Figure 2, immersed in the salt solution t0 a dep-th cf'about111/q"'and;mountedwith angle of divergence being aboutv 90. The belts,however, may diverge at an angle Abetween 45" and 180 although thepreferredangle isfbetweenBO" and 'Thesebelts, spaced 52 apart,Were'moved co-directionally'at about 7 per hour with the syrup as itadvanced with the salt solution, the belts retaining the syrup on thesurface of the salt solution so that asheet of about 50 in Width wasformed. Theimethyl Vm'ethacrylatesyrup wasV distributed onto thesurfaceof magnesium chloride'solution at the rate of about cc. per minute fromnozzle -3a'having an inside diameter of about 1/8". The'point ofdistribution of the'syrup onto the salt-solution was on they center linebetween the syrupretaining'beltsat a point' from the point'of divergenceof the belts. Thesalt solution wascirculated concurrently with the-syrupat about "l" per'hourandwas cooled and ltered :in'a cont-inuous fashion.Under ther-foregoing conditions a polymeric sheet, having a thickness ofabout 1/5",

`was formed.

The glass coveringr 5 of the Monel metalftray A' permitted themaintenance of an atmosphere oi'nitrogen above the lsurface of the`poly-merizing '-mass. Aglass, or other transparent material, that`does'not absorb unduly ultraviolet light-should, of course, be used,for the light bank B above cover A is used to induce polymerizationofthe monomeric ester. A stream of nitrogen, at the rate of from' to -25liters 4and preferably about 15 liters lper minutefwas passed-over thepolymerized portion of the sheet from a point 4 from the end of theunit, While the solution was maintained at a temperature of about 15 C.by a cooling jacket, not shown, beneath the tray. The inlet temper-`ature of the syrup was about 15 C., and'during polymerization thetemperature of the polymerizing'layerrose to a maximum of about4-0"C.

of BL-360 lights, producing light of maximum intensity at approximately3600 Angstroms wave Vlength, was suspended above the casting tray.

Upon emerging from the irradiated zone the polyi merized methylmethacrylate sheet was substantially completely polymerized. Thesheeting produced was of particularly high clarity with smooth surfacesand was free of bubbles. The sheeting was classified as optical quality.

The example was repeated except that-a stagnant atmosphere of nitrogenwas maintained over the entire upper surface of the polymerizing mass.The resulting sheeting, however, was found to be highly pitted. Themethyl methacrylate vapor in the stagnant atmosphere attacks the solidsheeting and appears to beresponsible for the pitting. When suchsheeting is subsequently heat treated, pitting is highly accentuated andthe resulting sheet has a frosted appearance. Plasticized sheeting ismore vulnerable to pitting than is unplasticized sheeting but regardlessof composition of the sheet produced, surface pitting is eliminatedentirely by passing an inert gas, such as nitrogen, over the sheeting asdescribed.

Distribution of the polymerizable liquid should be carried out in such amanner that at the time the syrup contacts the retaining means thelinear speed of each should be substantially thev distribution should besufliciently removed from the syrup retaining belts to insure that thesyrup during polymerization, at least, will be moving at approximatelythe same linear speed as the retaining .belts at the time of contact. Itfollows that the closer the point of distribution Accordtherefrom.Furthermore, the point of distribution does not have tovbe onthecenterline between -the'retaining v,belts so long as itis not too closelto a retaining belt.- In general, it is preferred that the point ofdistribution be between 3 to 6 inchesV from the point of divergence ofthe belts and on the approximate center line between the belts. Combinedwith the optimum angle of divergence of the belts, distribution' of thepolymerizable liquid, preferably in a single stream, at the optimumpoint results in the formation of optical quality sheeting.

Although operation within the optimum conditions of the presentinvention produces polymeric sheeting, especially methyl methacrylateand styrene sheeting, of highest optical quality, the so-called opticalimperfections in the sheeting produced by operation outside of theoptimum conditions are usually not 'apparent by visual inspection. Thismeans that sheeting not classied as optical sheeting can be used forvarious applications, such as a diffusing means in uorescent lighting,painted signs, electroplating tanks, Vand in various ornamentalapplications such as handbag and umbrella handles, lamp stands, displaystands, etc. Optical sheeting,

`which is used in aircraft enclosures, lenses, etc.,

depending upon the production rate, the dimensions of the sheeting andgeneral conditions of polymerization. For optimum results the forwardmovement of the sheet of polymerizing ester, the iiow of supporting saltsolution and the advance of the belts are synchronized to the same rateof travel which may vary widely. In operating in equipment of the sizedescribed,

speeds between 5 and 10 feet per hour can be to the syrup retainingbelts, the greater the dif- Y ference between the linear speed of theretaining belts and the adjacent portions of syrup, and V the greaterthe shearing forces between the retaining belts and the adjacentportions of syrup. The degree of shear between the retaining belts andsyrup determines the number and intensity of longitudinal lines whichare formed in the polymerized sheeting. Depending on the intensity ofthe lines they may or may not be detectable by visual inspection. Inmost cases these lines are only detectable by observing a shadow patternof the sheeting.

In general it is preferred to distribute the polymerizable liquid at apoint on the center line between the retaining belts. As describedabove, this point must not be too close to either of them. On the otherhand, the point of distribution. should not be too far removed from thepoint of divergence of the belts, otherwise the polymerizable liquidbetween the point of distribution and the point of divergence of thebelts will tend to stagnate. This would result in syrup hold-up andgenerally would cause non-uniform polymerization. Experience has shownthat the point of distribution should be no closer than about 2" fromthe point or divergence of the retaining belt and no greater than about12" used. A y

Any inert gas,kthat isfnot an inhibitor of polymerization, and'which isinert toward the liquid polymerizable organic compound and the resultingpolymer thereof, may be used in area D in VVthe process.Besides'nitrogen, other inert gases which may be'used include helium,neon,ar

f gon, etc.

As stated, it is very important that the inert rial is held to aminimum. This is accomplished by iirst introducing the inert gas intothe unit by means of a perforated tube 1 with discharge openingspointing in the direction of the sheet exit of the unit. Since there isno other outlet for the gas at the inlet end of the unit, inert gas iiowis maintained in the direction of the sheet exit. In addition to theproper direction of flow, it is also important that the rate of flow ofthe gas is not excessive. This should be adjusted in accordance with thegeneral dimensions of the polymerization unit. An excessive rate of owwill tend to cause excessive evaporation of liquid Vpolymerizable'compound inthe polymerization v.portion of' the unit, rtsultingr in .theformation -of optical irregularities inthe resulting sheeting. On theother handfa very low:I rate of vapor. flow "will: be ineffectual inpreventing-pitting of. the -,sheeting.because of 1 thehigherconcentration of monomeric vapors present. Y

Only a--smallamount of a Water soluble poly- .merizationdnhibiton :suchfas-a nitrite, is required to. inhibitY thermal polymerization and/or.photopOlymerization of the liquid polymerizable organic compounddissolved in `the.aqueousfsalt solution. From 0.001%110 0.05%,based'upon the weight Vof :aqueousnsalt solution, of a nitrite isusually-sumcient, from'.0.v00ll% to 0.005% vbeing preferred.Concentrations `below about 0.001% are ineffectual,- and concentrationssubstantially Y above .0.05% are-unnecessary. When the concentra'tionofnitrite approaches from '0.5% to-1%, bubbles, -ca-usediby decompositionof the nitrite, form inthe-aqueous-salt solutionand rise intothefliquidpolymerizable compound; thereby causing` bubbles to l beformed-fin the finished sheeting. Infgeneral, itv iszbelieved that thenitrite ion actually `attaches itself l-to` monomer units `and therebyprevents themonomer units from polymerizing.' Regardless 'of the actualmechanism of inhibition, ionizalole nitritesV ofthe type -hereindiscussed inhibit polymerization of liquid, poly- --mer-iz'ablecompounds-dissolved in the aqueous fsalt solution, -butdo not inhibitpolymerization of the adjacent layer l `of liquid polymerizablecom-pounds.

Other icnizable"nitriteswvhichlmaybe used as *polymerization inhibitorslin 4aqueous salt solutions, in accordance with-the'process illustratedvin .Example l, are'thenalkali metal nitrites-such :as sodium, potassiumor lithium nitrites andthe alkaline earth metal nitriteasuch vas-calciumor barium nitrites or mixtures thereof.

' Theinvention': is Aparticularly useful .as applied fto 'the productionoflmethyl-,fmethacrylate .polymer .orstyrenepolymer sheets, butrnaybeused for theproduction ofsheets onany liquid polymerizable compound.-Mixtures ofmethylY methacrylateorr st-yrenewith otherpolymerizablecompounds in lesser proportions may be substituted for themethyl methacrylate or' styrene monomers'orsyrup. Other polymerizableliquids/such `as'-mf thyl,ethyl, propyl and butyl acrylates' andethacrylates, ethyL' propyl and butyl methacrylates, vinyl-chloride,vvinylidene chloride, methyl styrene, and-thev like, are examplesofothervethylenically unsaturated compounds to which the processisfiully applicable, although sheets of i8 such polymerizable `compoundsare not in great demand; Y

I claim.:

l. Ina process for the preparation of methyl methacrylate'polymersheeting wherein a layer of monomeric-methyl methacrylate is poured onamoving surface of an aqueous liquid saturated with a-salt other than anitrite, the ester being polymerized thereon to form a sheet, the stepwhich comprises inhibiting the polymerization of any monomeric methylmethacrylate dissolved in the saturated salt `solution by the presenceof from 0.001% to 0.05% based on theweight of the saturatedfaqueousliquid of a nitrite ci a metal of the group lconsisting of alkalimetals-and alkaline earth metals, the nitrite salts of which are solublein the saturated aqueous salt solution.

2. In a process for thepreparation of methyl methacrylate polymersheeting wherein a layer of monomeric methyl methacrylate is poured on amoving surface of an aqueous liquid saturated with magnesium chloride,the ester being polymerized thereon to forma sheet, the step -whichcomprises inhibiting the polymerizationA of the methyl methacrylate insaid solution by the presence, based on the weight of the aqueousliquid, of from 0;001% to 0.005% of a nitrite of a metal of the groupconsisting of valkali metals and alkaline-earth metals, the-nitritesalts of which Vare soluble -in the saturated aqueous magnesium chloridesolution.

3. In a process for-preventingdefects in methyl methacrylate polymersheeting of substantially optical quality, produced by pouring a layerof a monomer-polymer syrup of methyl methacry-late on the moving surfaceof an aqueous solution vsaturatedwith magnesium chloride, the' esterbeing polymerized thereon to form a sheet, the-step which'compriseshaving present inthe aqueous magnesium chloride salt solution from0.001% to 0.05%, based on the weight of the aqueous magnesium chloridesolution, of a nitrite of a metal REFERENCES CITED The followingreferences are o record in the ile of'this patent:

UNITEDv STATES PATENTS Number Name Date 2,537,969 Chynoweth et al. Jan.16, 1951

1. IN A PROCESS FOR THE PREPARATION OF METHYL METHACRYLATE POLYMERSHEETING WHEREIN A LAYER OF MONOMERIC METHYL METHACRYLATE IS POURED ON AMOVING SURFACE OF AN AQUEOUS LIQUID SATURATED WITH A SALT OTHER THAN ANITRITE, THE ESTER BEING POLYMERIZED THEREON TO FORM A SHEET, THE STEPWHICH COMPRISES INHIBITING THE POLYMERIZATION OF ANY MONOMERIC METHYLMETHACRYLATE DISSOLVED IN THE SATURATED SALT SOLUTION BY THE PRESENCE OFFROM 0.001% TO 0.05% BASED ON THE WEIGHT OF A METAL SATURATED AQUEOUSLIQUID OF A NITRITE OF A METAL OF THE GROUP CONSISTING OF ALKALI METALSAND ALKALINE EARTH METALS, THE NITRITE SALTS OF WHICH ARE SOLUBLE IN THESATURATED AQUEOUS SOLUTION.